A method of detecting the presence of specific proteins in cells or tissues. Fixed cells or tissue on a microscope slide, made permeable if necessary with a detergent, are reacted with a primary antibody to the specific protein to be assayed. The preparation is then treated with a secondary antibody that has been coupled to an enzyme and which is directed against the primary antibody (e.g., goat anti-rabbit antibody). The preparation is then treated with a chromogenicsubstrate. Microscopic examination reveals the presence of staining, and hence of the specific protein to be detected. See one of the Figures from GXD for an example.